The mammalian sperm flagellum produces a regulated locomotive force that allows the sperm cell to proceed through a set of complex environments in the female genital tract and penetrate the egg investments. However, little beyond morphological description is known about the control of mammalian sperm flagellar formation, stability, or movement. The P.I. has mapped and isolated an axonemal dynein heavy chain, Dnahc8, that is expressed at high levels in a testis-specific fashion in wildtype mice. The map position of Dnahc8 and its mode of expression in the testes of certain male-sterile recombinant lines of mice is completely consistent with the flagellar organization and waveform defects, known as "whipless" and "curlicue", respectively, displayed in an allele-specific manner by sperm from these recombinant mice. Thus, Dnahc8 is a strong candidate for the allele-dependent expression of the "whipless" and "curlicue" mutations. Because Dnahc8 expression appears to have profound effects on both sperm tail biogenesis and function, Dnahc8 offers a key to unraveling the mechanisms underlying mammalian sperm tail assembly and/or motility. In order to test the hypothesis that Dnahc8 is responsible for expression of "whipless" and "curlicue", and to begin to clarify the cellular mechanisms underlying these phenotypes, the P.I. will determine the subcellular location of Dnahc8 in both mutant and control testis and cauda epididymal sperm through immuno-light and electron microscopy, and isolate potential spermiogenic binding partners of Dnahc8 through two-hybrid analysis (Specific Aim I). Concurrent with the performance of Specific Aim I, the P.I. will directly test the postulated role(s) of Dnahc8 in sperm tail assembly and function via its targeted deletion from the mouse genome (Specific Aim II). In order to assess the potential significance of Dnahc8 to human male infertility, the P.I. will localize the human ortholog of Dnahc8 to a sub-chromosomal region within the human genome, and will ascertain if this ortholog demonstrates polymorphisms diagnostic of human male infertility related to sperm flagellar defects (Specific Aim III). These experiments will reveal information crucial to our understanding of the molecular basis of mammalian sperm tail development and function in fertilization, while contributing to our ability to diagnose and treat human male infertility.